Taliban militants in Pakistan have released a graphic video that appears to show the beheading of a Polish engineer they kidnapped last September.

Officials have not confirmed the authenticity of the footage, which shows Piotr Stanczak appealing to the Polish government to withdraw its troops from Afghanistan. Then, two masked men apparently cut off the hostage's head.

One of the masked men on the video says Stanczak was killed because the Pakistani government refused to release detained militants before a midnight Friday deadline.

A Taliban spokesman told reporters on Saturday that the group killed the Pole and will not turn over his body until their demands are met.

Gunmen kidnapped Stanczak and killed three Pakistanis who were with him on September 28 in Punjab province.

Separately, the United Nations says it is deeply concerned about the fate of a U.N. refugee agency official, John Solecki, who was kidnapped one week ago in southwestern Pakistan.

Gunmen ambushed the U.S. citizen and his Pakistani driver, Syed Hashim, in Baluchistan province as they were traveling to work in Quetta on February 2. Hashim was killed in the attack.

A U.N. statement Sunday says the world body is seeking additional information about a group called the Baluchistan Liberation United Front, which told local media it is holding Solecki.

The U.N. says Solecki has a medical condition that requires medication, and it says security concerns have hampered the delivery of vital aid in western Pakistan.

Attacks on foreign workers in Pakistan have increased in recent months.